1. Field of the Invention
The present invention relates to passive infra-red security sensors, and, more particularly, to a system whereby the zones of protection of such a system can be located.
2. Prior Art
One type of passive infra-red security sensor comprises an infra-red detector which is mounted behind a window defining an array of Fresnel lens segments. Each lens segment focuses infra-red radiation from a particular zone onto an infra-red sensitive element of the detector. The Fresnel lens segments may be arranged in several rows, each of which contains several segments arranged side-by-side. Such Fresnel lens arrays may readily be formed by moulding a suitable infra-red transmitting plastics material. U.S. Pat. No. 4,321,594 (Galvin) describes the use of such a Fresnel lens array in a passive infra-red security sensor.
Another type of sensor uses a multi-faceted or segmented mirror mounted so that each segment reflects radiation from a zone of protection onto an infra-red sensitive element. Both types of sensor may use detectors with one or more elements.
In setting up a passive infra-red security sensor, it is desirable to know the location of the zones of protection. This enables the most likely risk areas, such as doors and windows, to be adequately covered, and also allows the installer to set up the sensor so that it is not directed at zones which are most likely to be the source of false alarms. For example, where a sensor is installed in a house where a dog or cat lives, it may be desirable to ensure that no zones are so close to floor level that the animal can activate the sensor. It is also undesirable to have any heat source, which may change temperature rapidly, located in a zone of protection as this may cause false alarms.
One solution commonly used for the location of zones is the provision of a walk test lamp connected to the output of the alarm detector circuit. The walk test lamp is located on the front of the sensor itself and illuminates whenever the detector senses a fluctuation in received infra-red radiation sufficient to produce an alarm condition. Since the activation of the walk test lamp requires the installer to walk through the zones, it is not always possible to precisely locate any individual zone. The function of the walk test lamp is primarily to confirm that certain activities will generate an alarm condition.
Another solution to the problem of locating the zones is described in GB-A-No. 2 064 108 (Arrowhead). In that system a zone locater lamp is mounted close to but spaced from the infra-red detector. The Fresnel lens array is arranged such that when standing in a zone defined by a Fresnel lens segment of a first row, the zone finder lamp can be seen through a Fresnel lens segment in a second row. Such a system of zone location is effective, but places constrains on the design of the Fresnel lens array and, moreover, only allows the zones defined by the first row to be located and not those of the second row.
The present invention therefore seeks to solve the technical problem of locating the zone or zones defined by a Fresnel lens or all or any of the segments of a Fresnel lens array or a multi-faceted or single-zone mirror in a passive infra-red security sensor.